The invention relates to a method and apparatus for controlling the differential speed between rotating structural parts such as a centrifuge drum and a screw conveyor wherein the screw driving torque and the differential speed between the drum and screw conveyor are measured and are controlled as a relationship of each other according to a predetermined characteristic programmed into a computer. The characteristic is changed to a new characteristic in a unique manner.
Screw centrifuges of the type referred to are used for the separation of solids liquids mixtures into their solids and liquids components. In the structure used, a screw conveyor rotates within a rotatable cylindrical conical drum. The solids are deposited against the inner surface of the wall of the drum as a result of the centrifugal force and the solids are conveyed toward a solids discharge from the drum by means of a screw conveyor which is operated at a differential speed relative to the rotating drum. The solids discharge is located in the conical end of the drum, and the material to be separated is supplied to the opposite end.
With a given amount of mixture containing solids, a specific conveying rate is employed. The solids load on the drum requires a specific differential speed between the screw conveyor and the drum, and the driving torque for driving the screw also has a specific value.
It is usually an objective to obtain a low differential speed between the drum and screw so that as long as possible a dwell time of the mixture occurs within the drum to obtain optimum separation of the liquid component from the solids. However, due to fluctuations in the properties of the input mixture, there is a danger of closing, stopping or clogging the unit when the conveying rate remains constant when the mixture encounters an increasing solids content. In this case, an increased solids load on the centrifuge occurs which results in a rapidly rising screw torque. Due to an overload of the screw drive, this can lead to a complete standstill or clogging of the unit. In addition to fluctuations in the solid content, other fluctuations in other parameters can occur which influence the settling behavior of the solids and these must be compensated for in the operation of the separator.
One method for the control of the differential speed between a centrifuge drum and a screw conveyor rotating therein is disclosed in German Patent Application No. P 30 05 658.7 and U.S. application, Ser. No. 234,382, filed Feb. 13, 1981, the disclosure of which is incorporated herein by reference. In that application, the speeds of the elements are measured free of contact, and an actual value of differential speed is determined by means of a computer. At the same time the torque for driving the shaft of the screw conveyor is measured, and a value for the differential speed is formulated from this measured value of torque in accordance with a predetermined functional relationship or characteristic which is stored in the computer and which is particularly specifically related to the nature of the slurry. A controlled variable which influences the speed of the drive of the screw conveyor is derived from the repetitive error which is derived between the actual and rated values determined in this manner. This prior method makes it possible for an operation of the screw centrifuge in such a manner that the operating point determined from the measured values for the screw torque and the differential speed is automatically held to the prescribed characteristic in a coordinate system defined by the measured values. In that case where the system has reached the maximum possible differential speed, i.e., has been levelled, a computer controlled throttling or shutdown of the feed of the solids liquids mixture is provided.
An object of the present invention is to provide a method for controlling the differential speed in rotary centrifuges which renders possible an automatic operation and is particularly advantageous in obtaining flexibility with fluctuations in parameters relating to the solids liquids mixture. This object is achieved in that the position of the functional relationship or characteristic between the speed differential and torque is changed step by step in a coordinate system by means of a computer with upward or downward change of values beyond a predetermined limit.
In a simple case the functional relationship or the characteristic between speed differential and torque is a straight-line relationship in a coordinate system with the differential speed being plotted on the abscissa and the screw torque being plotted on the ordinate. Along the plot or curve representing the functional relationship or characteristic, there are end points or maximum values of speed and torque. These limiting or maximum values of the characteristic are functionally allocated to a specific slurry consistency. With change in slurry consistency, the characteristic is automatically changed in a unique manner. What is expediently strived for is a mean position of the operating point in an area of the characteristic which renders fluctuations within specific band widths or ranges in both directions without limiting the values of the characteristic being reach. Within the range or limits prescribed by the structural features of the screw drive, the present invention renders possible a computer controlled change to a new characteristic corresponding to an altered slurry consistency with said new characteristic being used as the basis for control with the new slurry. This new characteristic or relationship will guarantee stable operating conditions with respect to the working range of the centrifuge. Of course, other parameters derived from measurable properties of the solids liquid mixture supplied to the centrifuge can be used as parameters for choosing the characteristic.
In accordance with the principles of the invention, the differential speed is controlled in accordance with the characteristic determined from a measurement of the screw torque. In comparison with the method referred to above in Patent Application No. P 30 05 658.7, the range in which the rated value for the differential speed can fluctuate is expanded by the totality of possible characteristics which can be set by the computer. By so doing a control method is created which adapts itself to variations and fluctuations, and is capable of handling broad fluctuations of parameters of the solid liquid mixture to be treated in the centrifuge.
In further accordance with the features of the invention, when the upper limiting value of any one characteristic is reached, the maximum value of the differential speed is adjusted until the screw torque has dropped below a predetermined value before a new characteristic is set. This is achieved by a measure that when a disruption occurs, a new characteristic or a new stationary operating condition occurs in the shortest possible time. By this arrangement, the maximum possible differential speed is expediently retained until the solids load of the centrifuge has been reduced to a proper operating condition and regular operation is possible in accord with the new characteristic which has now been set.
When the machine operates and the load on the machine increases due to change in rate of feed or consistency of the liquids solids mixture, and the upper limiting value of a given characteristic is reached, the differential speed is increased step by step. For each step, the change of the screw torque is measured with each step and a further increase in differential speed results if there has not been a change in screw torque sufficient to cause a change to a new characteristic. Thus, during a specific time interval after each increased step, the change of the measured value for the screw torque is observed and a further increase of the differential speed is obtained until the screw torque drops to a predetermined value whereupon the mechanism shifts to a new characteristic or a new relationship between torque and speed differential. These objectives can be obtained in terms of the technology of the apparatus used, by using a differential quotient generator which supplies a measured value for the change in output signal of the screw torque and this causes a further increase in the differential speed.
Another object of the invention is to provide an improved control mechanism which automatically selects a curve characteristic of relationship between torque and speed differential in a step-wise fashion with a mechanism and method operating in a rapid manner when a maximum or minimum value for each given characteristic is obtained.
Other objects, advantages and features will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: